For some pollutants in the 1999 NATA, USEPA included in the estimated total average ambient air concentrations a contribution from what it calls “background.” These are concentrations of the pollutants that are not accounted for by the emissions inventory. They are attributable to long-range transport, unidentified emission sources, and natural emission sources. A detailed description of how USEPA came up with these “background concentrations” can be found on USEPA’s NATA web site.

USEPA added background concentrations to the total air concentrations for 20 pollutants. Twelve of these are of concern in New Jersey. For ten of those pollutants of concern, USEPA estimates background concentrations that are higher than their respective health benchmarks, even without local emissions from within New Jersey. These ten pollutants are:

Acetaldehyde

Ethylene dibromide

Benzene

Ethylene dichloride

Bis(2-ethylhexyl)phthalate

Formaldehyde

1,3-Butadiene

Methyl chloride

Carbon tetrachloride

1,1,2,2-Tetrachloroethane

Chloroform and perchloroethylene are the other two pollutants of concern with significant background concentrations, although the background concentrations by themselves did not exceed health benchmarks.

Comparing the 1999 NATA New Jersey statewide average background concentrations of these ten pollutants to their health benchmarks results in risk ratios greater than one. This is shown graphically in Figure 1.

To gain a better understanding of the significance of these background concentrations, we have compared the percent contribution of each source category (major, area, on-road mobile and non-road mobile, plus background) to the total statewide average risk ratio. Results are given in Table 1.

Table 1. Source Category Contributions to Risk Ratios

Pollutant

% Contribution to
Risk Ratio from

Total Average Risk Ratio

Major

Area

On-road Mobile

Non-road Mobile

Back-ground

Acetaldehyde

2%

4%

57%

11%

26%

5

Benzene

1%

9%

51%

15%

25%

13

Bis(2-ethylhexyl)phthalate

0.2%

0.1%

0%

0%

100%

4

1,3-Butadiene

0.1%

5%

40%

14%

40%

5

Carbon tetrachloride

0.2%

0.4%

0%

0%

99%

4

Ethylene dibromide

0.04%

0.01%

0%

0%

100%

18

Ethylene dichloride

2%

0.4%

0%

0%

98%

1.2

Formaldehyde

1%

5%

37%

15%

42%

28

Methyl chloride

0.3%

1%

0%

0%

99%

2

1,1,2,2-Tetrachloroethane

0.2%

0.4%

0%

0%

99%

4

From Table 1, you can see that background is responsible for over 98% of the predicted risk from bis(2-ethylhexyl)phthalate, carbon tetrachloride, ethylene dibromide, ethylene dichloride, methyl chloride, and 1,1,2,2-tetrachloroethane. For most of the other pollutants of concern, mobile sources are the primary contributors to risk. Major and area sources contribute less to the risk from these chemicals.

Figure 2 shows the risk ratios for the twelve New Jersey chemicals of concern with background contributions, and the amount of risk attributable to different sources.

Comparing this graph to Figure 1, we can see that the total risk ratios for acetaldehyde, benzene, 1,3-butadiene, and formaldehyde more than double when concentrations from other sources are added to background concentrations. Chloroform and perchloroethylene risk ratios exceed one when emissions from area sources are factored in. The emissions from these pollutants will require multiple control stategies if the risk ratios in the state are to be reduced. For the six pollutants whose primary source is “background,” there is little that can be done locally to reduce the exposures experienced by the residents in this state.